1. Field of the Invention
This invention relates generally to polymeric additives for lubricating oil compositions. More particularly, this invention relates to a lube oil additive having both viscosity index improving and dispersancy properties.
2. The Prior Art
The newer engines place increased demands on the lubricants to be employed. In the past a number of different additives have been added to lubricating oils to improve such properties as viscosity index and dispersancy. One such additive added to lubricating oils to improve viscosity index is a two-block copolymer having the general configuration A-B where A is styrene and B is hydrogenated isoprene. See generally U.S. Pat. Nos. 3,763,044 and 3,772,196. A VI improver having greatly improved mechanical shear stability is the selectively hydrogenated star-shaped polymer disclosed in U.S. Pat. No. 4,166,917 by Rudolf J. Eckert, entitled "Hydrogenated Star-Shaped Polymer". Significant reductions in cost can be made by employing a single additive that improves a number of lubricant properties. However, in attempting to improve more than a single lubricant property, care must be taken in not causing the deterioration of other properties. For example, by employing an oxidation step to attach polar groups to the polymer backbone in U.S. Pat. No. 3,864,268, the patentees have reduced lubricant stability by introducing cites for oxidative attack.
From U.S. Pat. Nos. 4,077,893 and 4,141,847 hydrogenated star-shaped polymers are known which have been modified resulting in products which possess both dispersant and viscosity index-improving properties when used as additives in lubricating oils. These products are prepared by reacting in a first step a selectively hydrogenated star-shaped polymer comprising a nucleus and outwardly extending arms of polymers or copolymers of dienes and monoalkenyl arenes with an alpha beta unsaturated carboxylic acid or derivative, and then reacting in a second step the resulting intermediate with an alkane polyol, e.g., pentaerythritol or a polyamine. However, the polymer preparation process according to the above two patents requires multiple processing steps which greatly increase the cost of the additive along with introducing possible problems with the resulting polymers.